The present invention relates to water thickening polymers which are useful for increasing the viscosity of an aqueous medium. More particularly, the present invention is concerned with surfactant-containing water thickening polymers having improved salt tolerance.
It is known in the art that the viscosity of an aqueous medium is increased by the addition of water soluble polymeric materials. Previously used thickening materials have included numerous nonionic and/or polyelectrolyte water soluble natural or synthetic polymeric materials, such as gums, sugars, polymers, and the like. Examples of thickening agents include polyacrylamide, acrylamide/sodium acrylate copolymers, sodium polyacrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharides, naturally occurring gums, such as guar gum, chemically modified gums, such as hydroxypropyl guar gum, and biopolymers, such as xanthans derived from biofermentation of carbohydrates.
Accordingly, many industrial applications exist for aqueous media thickened with polymeric materials having water thickening capability. Such a thickened aqueous liquid is particularly useful in connection with fluid drive oil recovery processes, mobility profile modification, processes for drilling, completing or working over wells, or like processes in which a thickened fluid is injected into or brought into contact with a subterranean earth formation.
Water-soluble polymeric materials that are polyelectrolytes are generally efficient as water thickeners. In such materials, recurring units that contain similarly-charged hydrophilic ionic groups cause the dissolved molecules to be extended (and thus to have a large hydrodynamic volume) due to the repulsion between the charged groups. Examples of such polyelectrolyte thickening materials include the Pushers (available from Dow Chemical Company) and the Cyanatrols (available from American Cyanamid Company), comprising partially hydrolyzed polyacrylamide polymers.
However, while ionic water-soluble polymers such as neutralized acrylamide/acrylic acid copolymer, sodium polyacrylate, polystyrene sulfonate and the like are more efficient thickeners in deionized water than their nonionic counterparts, their thickening ability is greatly reduced by the presence of electrolytes such as sodium chloride, calcium chloride and magnesium sulfate in the aqueous medium. Such electrolytes are present in the aqueous media employed in most industrial applications, particularly those requiring the use of ground waters in subterranean formations as in enhanced oil recovery.
One approach to improving the thickening ability of ionic water-soluble polymers is shown in European patent application Publication No. 57,875 A2, published Aug. 18, 1982, which discloses a water thickening composition comprising (1) a water-soluble polymer having pendant hydrophobic groups and formulated to contain ten weight percent external sodium dodecyl sulfate, and (2) a water-dispersible surfactant. As taught in this publication, the water-soluble polymer is preferably a neutralized copolymer of acrylamide, acrylic acid and a hydrophobic monomer such as alkyl methacrylate, especially dodecyl methacrylate. The water-dispersible surfactant utilized is preferably a nonionic surfactant, such as ethoxylated alkyl phenols and ethoxylated fatty alcohols.
Unfortunately, a major disadvantage of two-component polymer/surfactant systems, such as described in European patent application publication No. 57,875 A2, is the tendency of these physical mixtures to undergo chromatographic separation in subterranean oil formations.
U.S. Pat. No. 4,403,152 discloses an aqueous viscosification or thickening agent which is a nonionic copolymer of acrylamide and an alkyl poly(ethyleneoxy) acrylate. Although this patent describes these nonionic polymers as good viscosification agents in salt water, they require high concentrations to be effective. In addition, the presence of ionic charge groups on the polymer backbone renders the copolymer ineffective. By comparison, ionic polyacrylamides in general are efficient water thickeners at low concentrations and therefore are normally preferred in oil recovely applications.
As a result, there exists a need in the art for a one-component polyelectrolyte which not only is an effective water thickener in deionized water but also exhibits improved salt tolerance in brine solutions at a low polymer concentration.